In tubular goods for high performance applications, such as those used in the oil fields, tubings and casings are widely made in accordance with the standards of the American Petroleum Institute (A.P.I.). The A.P.I. standards specify, for different sizes and grades of pipe, standards and acceptable tolerances for thread pitch thread dimensions and taper, and other characteristics. The geometries of the collars into which the threaded ends of the tubings or casings are to be engaged are similarly specified. The A.P.I. standard configurations leave open a central area between the beveled nose ends of the opposing tubular sections. This is called the "J area" and establishes a discontinuity between the inner diameter of the tubing or casing and the larger inner diameter of the central section of the collar. This abrupt inner profile discontinuity allows corrosive gases and liquids to penetrate into the threaded region, and also induces turbulence in flowing matter which accentuates the effects of the corrosive substances, especially when solids are entrained or injected in the fluids that are in the production stream.
To improve the durability and corrosion resistance of tubing and casing, a number of systems have been developed. Perhaps the most used of these is the so-called "Dual Lined" configuration, while a large number of installations also employ durable plastic coatings on the inside of the tubing. In a third category, the tubular goods are made of fiberglass reinforced resin, typically epoxy, while in a fourth category of installation a thin lining of cement is applied. In all of these expedients, the discontinuity, turbulence and exposed surfaces in the J area still present problems. Consequently, with these systems and also with prior art unlined connections, many attempts have been made to overcome the failures and difficulties. One approach has been to coat the J area surface with the same material as the tubing or casing coating. Another has been to use resilient elements interspersed between the opposing nose ends, for the purpose of providing a surface across the J area which is flush with the inner diameter of the tubing or casing. However, this presents a number of problems and heretofore has seldom been satisfactory, because of the tolerance variations permissible in production tubing and casing. If the interposed resilient element is not adequately compressed between the nose ends when the connection is made up, then leakage paths are created that induce corrosion and can tend to dislodge the resilient element. This can cause plugging of the tubing an surface production equipment. If the resilient element is too tightly engaged on opposite sides after makeup of the coupling, it bulges out into the path of the fluid moving along the tubular goods. The resilient element is quickly eroded or dislodged, and any such inward bulge impedes the passage of downhole tools. The inner diameter dimensions specified by the A.P.I. standards must be maintained to guard against these conditions.
The threaded ends of tubular goods are the portions most exposed to damage during handling, stabbing and makeup of a string. Chipping and cracking of the coating on surface areas at or near the ends often occurs in normal use. Moreover, the stresses and deformation introduced during makeup sometimes introduce new cracks in the coating, or accentuate prior existing defects. Close and thorough inspection for minute pre-existing chips or cracks is not feasible under field conditions and is out of the question once the coupling has been made up.
Systems and methods for confronting these problems should be employable in the field in such manner as to achieve rapid makeup of tubing and casing strings while also providing assured protection against problems encountered in the J area of the couplings. They should reliably overcome the problems presented by standard A.P.I. couplings with 8 round or buttress thread under corrosive conditions.